Coordination defects in amorphous silicon and hydrogenated amorphous silicon: a characterization from first-principles calculations

We study by means of the first-principles pseudopotential method the coordination defects in amorphous silicon and hydrogenated amorphous silicon, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the 'electron localization function' allows us to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly threefold (T-3) and fivefold (T-5) coordinated defects. We found that electronic states in the gap can be associated with both kinds of defect and that in both cases the interaction with hydrogen can reduce the density of states in the gap.